River Sturgeons of the American Genus Scaphirhynchus: Characters, Distribution, and Synonymy

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186 Bailey aiul Cross Hanel } - 16 per cent in specimens more than 300 mm. long. Con - spicuous also is the gross variation in its size among fish of the sane approximate length. If relative length of the air bladder is apprais.. rl in terms of body length (Fig. 7), the overlap between the species is slight. Mea.virenient ratios. —In the preceding discussion we have seen that there are a considerable number of measurements in which the species of Scaphirh1pichus differ. With the exception of the length of [lie inner bariic I, however, the differences are not so great as to he nnutually exclusive. In other measurements some specimens fall in an overlapping area. In attempting identification on the basis of morphometric data alone, then, it is frequently- necessary- -and al- wa }-s a �• ise pro-,mtion —to check several measurements. .Judicious combination of two or more characters into a single expression often makes possible the elimination of overlap in com- paring species. This has been readily- accomplished in Seaphirhun- chus through use of ratios so chosen as to combine proved difference; .,Ie il urder to minimize the effect of pronounced ailo- metric change the data are separated into two size classes. The ratios numbered 2 and 4 in Table IV have already been mentioned. Of the 9 ratios tabulated, there is no overlap (within a size class) in 7, and only a slight overlap occurs in 2 (numbers 7 and 9). dr''- tional ratios could be used, but those given are adequate to provi - a cie+ r sion of the principal inorphoinetric differences notes. By plotting ratio 6 against ratio 8 it has been possible to develop a visual expression of specific difference involving 4 of the most valu- able contrasting measurements: head length, inner - barbel length, dis- tance from snout tip to outer barbel. and height of the tenth lateral plate ,F S'!• Most of the significant morphometric characters in Scaphirhynchus are discernible in a ventral view of the head. These are illustrated in Figure 9, in which the drawings are based on mean values from Tables II and III. The details of structure are from individual speci- mens. Comparison with these figures and use of the ratios should facilitate quick and accurate identification. Other Characteoe Ribs. —In their characterization of Parascaphirhynchus albtts, Forbes and Richardson (1905) emphasized the number of ribs as a